1. Field of the Invention
The present invention relates to a self-cooled motor, and more specifically to an improvement in a motor including a cooling blade configured to rotate together with a shaft.
2. Description of the Related Art
A common microwave oven includes a stirring fan arranged to stir air inside a heating chamber. The stirring fan is arranged inside the heating chamber, and is driven by a stirring motor arranged outside the heating chamber. The stirring motor is attached to an outside of a wall surface of the heating chamber with a shaft thereof projecting into the heating chamber. Thus, the stirring motor is in such an environment that heat is easily transferred from the heating chamber to the stirring motor, and therefore, an AC motor which is excellent in heat resisting property is used as the stirring motor.
The temperature of an interior of the heating chamber of the microwave oven typically becomes 300° C. or higher. On the other hand, a common magnet has a heat-resistant temperature of about 150° C., and decreases in coercive force when the temperature of the magnet exceeds about 130° C. Moreover, electronic components have heat-resistant temperatures still lower than the heat-resistant temperature of the magnet. Therefore, a brushless DC motor including a magnet and electronic components cannot be used as the stirring motor, and instead, an AC motor is typically used as the stirring motor.
With the AC motor, it is impossible to perform control of a rotation rate and a rotation direction more finely than with the brushless DC motor. Therefore, known microwave ovens have a problem in that it is impossible to finely control the rotation rate and the rotation direction of the stirring fan to prevent uneven cooking and an uneven temperature distribution. In other words, the known microwave ovens have a problem in that it is difficult to improve functionality by employing the stirring fan.
It is then conceivable to employ a brushless DC motor including a cooling mechanism as the stirring motor to improve the functionality of the microwave oven. A variety of techniques have been proposed with respect to the cooling mechanism of the brushless DC motor (see JP-A 2008-154369, JP-A 2000-184644, JP-UM-A 62-178777, and JP-A 2000-050575).
However, the cooling mechanism disclosed in each of the aforementioned patent documents is designed to discharge heat generated inside the brushless DC motor to an outside, and is therefore unable to sufficiently cool the brushless DC motor when the brushless DC motor is arranged in the vicinity of an external heat source, such as the heating chamber of the microwave oven.
For example, a motor described in each of the aforementioned patent documents includes a cooling mechanism which causes a blade arranged on a side opposite to a side where an attachment surface exists to rotate to send air into the motor from the side opposite to the side where the attachment surface exists. This cooling mechanism is unable to sufficiently reduce the likelihood that a high-temperature air in the vicinity of the attachment surface will flow into the motor. Moreover, because radiant heat from the attachment surface, which has a high temperature, is not taken into consideration, this cooling mechanism is unable to exhibit sufficient cooling performance.